What exactly does a `symmetric modal relation' look like? Ney bases her account on a symmetric modal relation that she refers to as determination, defined as `law-governed co-instantiation,' which could be expressed quantitatively in the form \(p(A = B) = 1\) . More generally, we could also imagine other `law-governed' modal relations expressed in the form \(p(A = B) = x\)  for any \(x \in (0, 1)\) . These sorts of modal relations are less familiar to us than cause-effect relations, as evidenced by the fact that there does not seem to be any word for them in ordinary non-technical language, but they are in fact ubiquitous in science. For in Cartwright's terminology, many laws that appear in science are not causal laws but rather `laws of association ... (which) tell how often two qualities or quantities are co-associated'[1]} - and indeed this observation was indeed another of Russell's arguments for his claim that causation does not appear in fundamental science[2]}. For example, consider a relation like the Maxwell-Faraday equation, \( \nabla \times E = -\frac{ \partial B}{\partial t}\) . This equation is not directed: it is not the case that the state of the electric field causes the magnetic field to have a certain rate of change or vice versa, since there is no fact of the matter about which `comes first.' But nonetheless the relation between \(E\)  and \(\frac{ \partial B}{\partial t}\)  is not just `bare correlation' - it is an instance of Ney's `law-governed co-instantiation,' which is to say it is nomically necessary that the two must stand in this symmetric relationship. Moreover, although we tend to think of dynamical laws as being intrinsically directed, Ismael emphasizes that dynamical equations are not really different in form from these co-instantiation laws: `Dynamical laws ... are constraints on the relationships between states at different times, but ... there is nothing in the law itself to say either determines the other.'[3]} So in fact, there are good reasons to suppose that we should think of dynamical equations in the same way as equations like the Maxwell-Faraday equation - they simply express a symmetric, undirected modal relationship between states at different times. Thus realism need not be tied to causal explanation, as we can offer a legitimately realist account of fundamental physics which replaces causation with symmetric modal relations linking events at different times.
